Atmosphere is a very complex, time dependent turbulent medium for transmission of optical signals. Laser beam propagation in turbulent atmosphere can be accompanied by random beam wander (changing central direction of laser beam propagation) and strong aberrations with formation of inhomogeneous, also called speckled, spatial structure of the laser beam. Cross section of intensity profile of the speckled beam consists of randomly distributed bright spots (“speckles”) divided by regions with low or zero light intensity. For typical horizontal path and space communication uplink scenarios, many speckles can form within the transmitted beam spot.
Time variation of the atmosphere parameters, such as local temperature and density of air, causes changes in the speckle distribution that appears as random “boiling” of speckle pattern, in other words, random movement of points of intensity maximums and minimums across the beam. This represents the scintillation effect, which appears as temporal fluctuation of laser beam intensity spatial distribution. That translates into fluctuation of a signal power acquired by a given aperture receiver. In particular, if the aperture is comparable to speckles size, received power can drop significantly for time intervals when the receiver happens to be between speckles. This effect is referred to as signal fading. Impact on signal fading by scintillation is mostly pronounced when speckle size is larger than the receiving aperture, which is practically always the case for long range ground to satellite uplink communication channel.
The random beam wander in turbulent atmosphere may also lead to significant reduction of the receiver signal power acquired when the laser beam shifts as a whole far enough outside the receiving aperture. This beam wander effect differs from temporally variable speckle motion driven by scintillation process. Its dynamics is usually much slower and impact from it can be minimized by adaptive adjustment of the laser beam pointing direction. Pointing adjustment cannot, unfortunately, reduce signal fading due to speckles boiling within the beam spot.
Fried parameter is a commonly accepted measure of the quality of optical transmission through the atmosphere due to random inhomogeneities in the atmosphere's refractive index induced by variations in temperature and thus density. The Fried parameter have units of length and is typically defined as the diameter of a circular area over which the RMS of atmosphere passage induced wavefront deviations from plain surface normal to original pointing direction is equal to 1 radian. Fried parameter defined this way integrates together two effects, wavefront tilts due to beam wander, and randomized variations of the wavefront shape at lateral scales smaller than the beam size.
Free space laser communication (lasercom) systems are currently being used for transfer of information between two points in atmosphere and/or space which utilize superimposing the information signal as time modulation onto directed laser beams serving as information carriers that propagate between those two transmitting and receiving points. Signal fading, an effect of random signal disappearing or reducing its power below a certain threshold level, is a well-known problem that limits performance of any electromagnetic waves based communication system. In particular case of lasercom systems operating through turbulent atmosphere, which are usually supplied by tracking system, signal fading occurs mostly due to scintillation phenomenon.
Other methods have been proposed to minimize signal fading due to scintillation effect in atmosphere for lasercom systems. These proposed methods include automatic gain control on a receiving amplifier, using large receiving aperture or multiple dispersed receiving apertures, and employing adaptive optics and phase conjugation. However, all of these proposed methods are not only technically complex and expensive to implement, but also, far from solving the signal fading problem, and usually add extra noise to communication signal. They also have limited capability in application to long communication links between ground and satellites.